High temperature resistant sheet materials are well known to the art and are used under various high temperature conditions. These sheet materials are known in the art as "papers", since they are often made by methods similar to paper-making methods, although the thickness thereof can be up to one-half inch or more. The papers are often used as protective lining against high temperature, but they have other uses, such as a high temperature filtering medium. Most often, these papers are made by laying staple fibers into a matt and consolidating the matt into a paper, although other processes may be used. The staple fibers used in making these papers are heat resistant inorganic fibers such as glass, metal or ceramic fibers. By virtue of the laying operation, the staple fibers are randomly oriented and, with consolidation, are interlocked together into the form of a shape sustaining paper having two lateral surfaces.
The paper has considerable structural integrity due to the random orientation and interlocking of the fibers, but that structural integrity is quickly deteriorated by mechanical action on the paper, such as abrasion.
To avoid the loss of structural integrity during mechanical action, e.g. abrasion, the papers often have a binder applied thereto. These binders take various forms, but generally speaking the binders are organic polymers such as phenolics, acrylics and epoxies. The binders serve to improve the structural integrity of the papers during manufacture and fabrication of the papers into products.
However, the binders of these papers, while quite satisfactory at ambient temperatures, will begin to lose the binding effect at elevated temperatures, with a concomitant loss of structural integrity of the papers. At even higher temperatures, and the temperatures at which these protective papers are normally used, the binder will burn away and the structural integrity of the paper will again depend entirely upon the interlocking of the fibers. This is quite satisfactory for papers which do not experience considerable mechanical action in use, since once the paper has been mechanically manipulated, conformed and fitted to the configuration of the particular apparatus in which it is used, it is most often held in place by the apparatus itself and substantial independent structural integrity of the paper itself is not required.
As an example of the foregoing, these papers are often fabricated into high temperature resistant gaskets. Thus, once the paper has been manipulated and configured into a proper shape, etc. for the gasket, and installed, there is no need for higher structural integrity, since the gasket will be held in place by the apparatus itself. As another example, these papers are often fabricated into insulating liners for stationary kilns. Once the papers have been cut, etc. and fitted to the kiln walls, the papers are held in place by the fire bricks lining the kiln and no higher structural integrity is required. Thus, in both of these examples it is of no substantial concern that the binder burns away in use and the structural integrity of the papers is materially reduced.
On the other hand, serious problems have been faced in the art when these papers are used under conditions where the papers are subjected to substantial mechanical action after the binder has been burned away. For example, these papers are used as a protective back-up thermal insulator between the shell and the fire bricks of high temperature rotary kilns. The usual binders of these papers are quite sufficient for allowing the papers to be cut, configured, and installed on the kiln shell and to sustain the mechanical action of placing the fire bricks on the installed paper. However, with operation of the kiln, the natural movement of the bricks, relative to the shell of the kiln during rotation of the kiln, causes substantial mechanical action on the paper. Once the binder has burned away, the structural integrity of the paper is so reduced that the mechanical action of the bricks, relative to the shell, will eventually destroy the paper and the protective value thereof.
As another example, these papers are used in high temperature gas filtration. Again, the binder is quite satisfactory for fashioning the filters, e.g. flat filters or bag filters, but after the filter is used in high temperature operation, the binder burns away and the structural integrity of the papers results solely from the interlocking of the inorganic fibers. This remaining integrity is not sufficient to withstand the flexing of the filters, and filters made of these papers will ultimately deteriorate, tear and otherwise become unserviceable.
Such material action on the papers, as described above, is referred to as an abrasive action, since it causes the interlocked fibers to move relative to one another and literally abrade each other, causing deterioration of the interlocking, and, hence, deterioration of the structural integrity. This abrasion, however, may be the result of a number of different induced mechanical actions, including flows of fluid across or through the papers, flexing of the papers, relative movement of apparatus next to the papers, and the like.
The art has sought solutions to the problems of these papers being used in such abrasive, high temperature environments, i.e. where the binder is burned away and the abrasive action deteriorates the structural integrity of the papers. One approach in the art has been the search for improved binders, with the hope that the binders would sustain higher temperatures for longer time periods and, thus, preserve the structural integrity of the papers for longer time periods. Some success in this approach has been achieved, but since most of the binders are organic-based binders, e.g. polymers, those binders cannot withstand the higher temperatures to which these papers are subjected and no organic binder, currently available, is capable of substantially increasing the life of these papers in abrasive, high temperature environments.
Another approach in the art has been that of increasing amount of binder in order to prolong the life of the paper, but increased amounts of binder significantly decrease the workability of the paper and particularly in fitting those papers to irregularly shaped apparatus. Further, the increased amount of binder does not substantially extend the life of the paper in abrasive, high temperature environments.
It would be, therefore, of considerable advantage to the art to provide such papers, of the nature described above, wherein the structural integrity of the papers can be largely maintained at higher temperatures where the binder burns away from the papers and where the papers are subjected to abrasion, as described above.